Page 23 - Well Logging and Formation Evaluation

Basics 13

1
8 / 2≤ hole section:
GR/resistivity/density/neutron
Dipole sonic/formation imager
Pressure/fluid sampling (sampling dependent on oil being encountered)
VSP
Sidewall samples

Note that it is not usually necessary to specify that the SP will be run,
since this service is usually provided free and the log will be included in
the first toolstring in the hole by default. Likewise, thermometers are
usually run in the toolstring as standard, and the maximum temperature
recorded is included in the log header. Prior to the actual logging job in
each section, a program is usually sent to the rig with the following more
detailed specifications:

• The actual mnemonics of tools to be run (dependent on the contractor)
• Intervals to be logged if different from the total openhole action
• How the tools are to be combined to form the individual toolstrings
• Data transmission/delivery requirements

For the so-called conventional logs (i.e., GR, resistivity, sonic, density,
neutron), it is not usually necessary to be very specific, since the company
will have already established the tool parameters via generalized guide-
lines, as discussed in Section 1.3 above. However, the type of resistivity
tool to be used will depend on the drilling mud in the hole and the resis-
tivities expected to be encountered. While only induction tools may be
run in OBM, the optimum tool to be run in WBM will depend on the ratio
of the mud filtrate resistivity (R mf) to the formation, or water, resistivity
(R w). As a rule of thumb, an induction tool is preferred if the ratio of R mf
to R w is more than 2. However, laterolog tools tend to be more accurate
in highly resistive formations (resistivity at room temperature >200
ohmm) and are inaccurate below about 1ohmm. Induction tools, on the
other hand, become saturated above 200ohmm but are more accurate in
low-resistive formations. For formation imaging, resistivity tools cannot
be used in OBM, although they are definitely preferred in WBM. When
OBM is used, it is necessary to use an ultrasonic device.
Usually the stations to be used for pressure/fluid sampling, VSP, and
sidewall sampling will be dependent on the analyses made on the first
run(s) in the hole. These stations may in some cases be picked on the well-
site by the company’s representative but are usually determined in the